Dissolvable magnesium alloy

ABSTRACT

A dissolvable magnesium alloy can be used for components of a downhole tool. The dissolvable magnesium alloy can be dissolved completely and controlled at a dissolving rate so as to be compatible with downhole operations, including hydraulic fracturing operations. The alloy includes nickel at 0.04-0.4% by weight and the balance of magnesium. The alloy is dissolvable in KCl at 2.1% by weight and 95 QC with a dissolving rate in a range of 10-100 mg/cm2/hr, yield strength in a range of 18-37 ksi, ultimate tensile strength in a range of 29-47 ksi, and elongation in a range of 8-40%.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a material composition in the oil and gas industry. More particularly, the present invention relates to dissolvable metal alloys to form components of downhole tools. Even more particularly, the present invention relates to a dissolvable magnesium alloy for components in hydraulic fracturing operations.

Background

Downhole tools are commonly used in oil and gas production. A borehole is drilled through a hydrocarbon bearing formation, and downhole tools, such as plugs and sleeves are positioned along and within the borehole. The plugs close and open portions of the borehole so that zones may be selectively isolated. A plug can include at least one dissolvable metallic component. As an assembly, the plug must hold a pressure differential around 7.5 ksi. A sleeve opens and closes to make the fluid connection between the borehole and the formation. The downhole tools work to isolate and connect the zone for various operations to prepare and produce the hydrocarbons from the formation. When the operations are complete in the zone, components of the downhole tool or even the entire downhole tool may require removal. For example, a dissolvable frac ball set in a plug to trigger a seal may be removed by injecting a solvent targeted to the dissolvable frac ball so that the seal is removed. After the fracturing operation, the components dissolve in the wellbore fluid, typically a potassium chloride brine. Alternatively, the entire plug may be removed.

Dissolvable alloys were developed for the manufacture of downhole tool components in the oil and gas industry. There are mainly two types of metallic dissolvable alloys: magnesium and aluminum based alloys. These alloys may be cast and mechanically worked in a variety of manners, including but not limited to vertical direct chill casting, vacuum induction melting, and extrusion.

The disclosure of dissolvable metal alloys, and particularly, dissolvable magnesium alloys are known in the prior art intended for a variety of conditions. US Publication No. 20160168965 published on 16 Jun. 2016 for Marya, U.S. Pat. No. 8,425,651, issued on 23 Apr. 2013 to Xu et al, US Publication No. 20140286810 published on 25 Sep. 2014 for Marya, US Publication No. 20180238133 published on 23 Aug. 2018 for Fripp et al, US Publication No. 20190032173 published on 31 Jan. 2019 for Sherman et al, US Publication No. 20190055810 published on 21 Feb. 2019 for Fripp et al, US Publication No. 20190271061 published on 5 Sep. 2019 for Tang et al, U.S. patent Ser. No. 10/081,853 issued on 25 Sep. 2018 to Wilks et al, U.S. Pat. No. 9,757,796 issued on 12 Sep. 2017 to Sherman et al, and US Publication No. 20160256091 published on 20 Aug. 2019 to Cho et al, disclose dissolvable magnesium alloys.

It is an object of the present invention to provide a dissolvable magnesium alloy.

It is another object of the present invention to provide a dissolvable magnesium alloy for components of a downhole tool.

These and other objectives and advantages of the present invention will become apparent from a reading of the attached specification.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention include a dissolvable alloy for components of a downhole tool. The assembly of a downhole tool with a dissolvable metallic component, which comprises a dissolvable alloy, having from about 0.04 to about 0.4 wt % nickel and the balance of magnesium, holds a pressure differential around 7.5 ksi and dissolves in a wellbore fluid after downhole operation. The alloy is dissolvable in KCl at 2.1% by weight and 95 QC with a dissolving rate in a range of 10-100 mg/cm²/hr, yield strength in a range of 20-40 ksi, ultimate tensile strength in a range of 25-45 ksi, and elongation in a range of 7-40%.

Optionally in any embodiment, the alloy may comprise about 0-20% lithium by weight, about 0-15% gadolinium by weight, about 0-15% yttrium by weight, about 0-2% copper by weight, about 0-2% zirconium by weight, about 0-15% aluminum by weight.

Optionally in any embodiment, the alloy may comprise up to about 10 wt % total of other elements. which comprises one of manganese, neodymium, cerium, calcium, iron, bismuth, indium, or silver.

Optionally in any embodiment, said copper is about 1.4 wt %, said gadolinium is about 3.1 wt %, said nickel is about 0.15 wt %, and said yttrium is about 4.0 wt %.

Optionally in any embodiment, said copper is about 1.47 wt %, said aluminum is about 10.1 wt %, said zinc is about 0.45 wt %, said nickel is about 0.15 wt %, and said manganese is about 0.16 wt %.

Optionally in any embodiment, said copper is about 0.4 wt %, said nickel is about 0.04 wt %, and said aluminum is about 0.5 wt %.

Optionally in any embodiment, said gadolinium is about 3.1 wt %, said copper is about 0.4 wt %, said nickel is about 0.04 wt %, and said aluminum is about 0.5 wt %.

Optionally in any embodiment, said copper is about 0.4 wt %, said nickel is about 1.4 wt %, and said aluminum is about 5.6 wt %.

Optionally in any embodiment, said gadolinium is about 1.56 wt %, said zirconium is about 0.4 wt %, said nickel is about 0.04 wt %, and wherein said zinc is about 3.88 wt %.

Optionally in any embodiment, said lithium is about 11 wt %, said gadolinium is about 1.0 wt %, said yttrium is about 0.6 wt %, said nickel is about 0.4 wt %, said copper is about 0.2 wt %, and wherein said zinc is about 3.3 wt %.

In another embodiment, an alloy may comprise about 0-20% lithium by weight, about 0-15% gadolinium by weight, about 0-15% yttrium by weight, about 0-2% copper by weight, about 0-2% zirconium by weight, about 0-15% aluminum by weight, about 0.04% to about 0.4% nickel by weight; the balance of magnesium (Mg) and inevitable impurities, so as to be dissolvable in KCl at about 2.1% by weight and about 93° C. with a dissolving rate in a range of from about 10 to about 100 mg/cm²/hr, yield strength in a range of from about 25 to about 37 ksi, ultimate tensile strength in a range of from about 35 to about 45 ksi, and elongation in a range of from about 10 to about 19%.

Optionally in any embodiment, the copper to nickel ration is within the range of about 50:1 to about 0.1:1 wt %.

Optionally in any embodiment, said range of aluminum is about 7 to about 12 wt %, particularly when the range of aluminum to copper is in the range of about 3.5:1 to about 60:1 wt %.

Optionally in any embodiment, said range of zinc is about 0.5 to about 3 wt %, particularly when the range of copper to zinc is in the range of about 0.07:1 to about 4:1 wt %.

In further embodiment, an alloy may comprise about 0.04% to about 0.4% nickel by weight; about 0.2% to about 2% copper by weight; and the balance of magnesium (Mg) and inevitable impurities, wherein the alloy yields strength in a range of from about 25 to about 37 ksi, ultimate tensile strength in a range of from about 35 to about 45 ksi, and elongation in a range of from about 10 to about 19%.

Optionally in any embodiment, the alloy yields strength in a range of from about 25 to about 37 ksi, ultimate tensile strength in a range of from about 35 to about 45 ksi, and elongation in a range of from about 10 to about 19%.

Optionally in any embodiment, the alloy may further comprise about 0-20% lithium by weight, about 0-15% gadolinium by weight, about 0-15% yttrium by weight, about 0-2% zirconium by weight, about 0-15% aluminum by weight.

Optionally in any embodiment, the alloy may further comprise up to about wt % total of other elements. which comprises one of manganese, neodymium, cerium, calcium, iron, bismuth, indium, or silver.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating a comparison of stress and elongation for an embodiment of the alloy of the present invention with additional additives according to one embodiment. All values are as-extruded with no heat treatments.

FIG. 2 is a graph illustrating the yield strength, ultimate tensile strength, and ductility at room temperature for alloys 1 through alloy 6.

FIG. 3 is a plot of the mechanical properties of alloy 6 as a function of temperature. The mechanical properties are stable as a function of temperature.

FIG. 4 is a plot of the yield strengths normalized to room temperature for alloy 1 through alloy 6 as a function of temperature. The plot shows a prevalent decrease in properties with increasing temperature. Rare earth elements such as gadolinium stabilize mechanical properties with increasing temperature. The plot teaches for the two embodiments with gadolinium (alloys 4 and 5), there is a loss greater than 15%. The addition of yttrium maintains the maximum property loss to 10%.

FIG. 5 is a graph illustrating an ultra-high ductility magnesium, Alloy 7. The addition of lithium to the material greatly increased the ductility at the expense of mechanical strength.

FIG. 6 is a graph illustration contrasting the ductility of alloy 7 with other embodiments in the disclosure.

FIG. 7 demonstrates the unique performance of alloy 7 over two other high elongation embodiments, alloy 6 (left) and alloy 5 (center). These show the standard failure patterns of magnesium in compression due to the HCP crystal structure. The lithium addition overcomes the limitations of the magnesium HCP crystal structure.

FIG. 8 is a graph illustration as-cast dissolution rate of six embodiments of the alloy of the present invention at 95 C in a range of salinities. The dissolution rate range shows the chemistries cover applications where either quick or slow dissolution is needed.

FIGS. 9 a through 9 g present a magnified view of a microstructure of an embodiment of the alloy of the present inventions with different additives at a range of magnifications. The figures teach the resultant phases and microstructures that generate the measured properties.

FIG. 10 teaches the change in microstructure from the addition and/or subtraction of gadolinium from the alloy.

FIG. 11 teaches the impact yttrium has on microstructure.

FIG. 12 a through 12 d show phase diagrams that are used to select the fractions of alloying elements to be added to an alloy.

FIG. 13 is an X-Ray Diffraction measurement of phases for alloy 3 showing the creation of beneficial phases in an embodiment of the alloy of the present invention.

FIG. 14 is an X-Ray Diffraction measurement of phases for alloy 6 showing the creation of beneficial phases in an embodiment of the alloy of the present invention.

FIG. 15 is a graph illustrating a CALPHAD simulation of the embodiment of the alloy of the present invention in FIG. 5 , showing the mole fraction of phases formed during casting.

FIG. 16 is a graph illustrating comparison of alloy 4 to an alloy using the same weight percentage rare earth, but substituting neodymium for gadolinium.

DETAILED DESCRIPTION OF THE INVENTION

Before the description of the embodiment, terminology, methodology, systems, and materials are described; it is to be understood that this disclosure is not limited to the particular terminologies, methodologies, systems, and materials described, as these may vary. It is also to be understood that the terminology used in the description is for the purpose of describing the particular versions of embodiments only, and is not intended to limit the scope of embodiments. For example, as used herein, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. In addition, the word “comprising” as used herein is intended to mean “including but not limited to.” Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art.

Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as size, weight, reaction conditions and so forth used in the specification and claims are to the understood as being modified in all instances by the term “about”.

Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

As used herein, the term “about” means plus or minus 10% of the numerical value of the number with which it is being used. Therefore, about 50% means in the range of 45%-55%.

Magnesium alloy is an ahoy based on magnesium and some other additional elements. It has the following characteristics: low density (about 1.8 g/cm³), high specific strength, high specific elastic modulus, good heat dissipation, good shock absorption, better impact load resistance than aluminum alloy, and good resistance to organic and alkali corrosion, Magnesium alloy has a wide range of applications in various industrial fields, mainly used in aviation, aerospace, transportation, chemical, rocket, oil and gas industries and other industrial sectors. Magnesium is the lightest metal in the practical applications. The specific gravity of magnesium is about ⅔ that of aluminum and ¼ that of iron, Magnesium alloy has high strength and high rigidity. On the other hand, magnesium alloy is chemically active among existing materials and can be used in industrial fields where structural materials are required to be degradable.

Although the chemical properties of magnesium alloys are relatively active, the reaction rate of magnesium with a medium such as water, aqueous solutions and water-oil mixtures is extremely slow at normal temperature. The main reason is that the magnesium hydroxide formed by the reaction can prevent further reaction between magnesium and the medium. Even if the magnesium alloys are heated to the boiling temperature of water, only a very slow reaction can be observed. Because the reaction rate of conventional magnesium alloy with medium is low within a certain temperature range and the controllable range is narrow, it cannot meet the demands of industrial applications. Thus, for the manufacture of structural and functional integrated components in industries such as oil and gas sector, there is a great need for an improved alloying process that would enhance the rate of chemical reaction between magnesium alloy and the medium, while maintaining the high strength of the magnesium alloy.

The present invention presents a magnesium alloy which would destroy the continuity of magnesium hydroxide formed during the reaction between magnesium and a medium, thereby accelerating the reaction between magnesium and the medium. The medium could be aqueous solutions such as fresh water, pond water, lake water, salt water, brine water, produced water or flow back water and their mixture with crude oil etc. In one embodiment, the chemical reaction can be: Mg+2H₂O→Mg(OH)₂+H₂ (gas)

By adjusting the proportion of each element in the magnesium alloy, the reaction rate of the magnesium alloy with the medium can be regulated, resulting in a relatively wider controllable range, and the magnesium alloy material is flexible, so that the magnesium alloy meets the application requirements of the industrial sector such as oil and gas industry.

The mechanical properties such as tensile strength and yield strength of magnesium alloy are improved by adding gadolinium and yttrium to the magnesium alloy. Tensile strength is the resistance of a material to breakage under tension and it is usually obtained by the stress-strain curve. The unit is usually in MPa or KSI etc. Elongation is the amount of extension of an object under stress upon breakage, usually expressed as a percentage of the original length. In the application in oil and gas industries, such as frac balls, frac plugs or frac seats, not only has the magnesium alloys to be dissolved in a medium, but also the alloys need to have higher mechanical strength to withstand the high pressure and high temperature scenario.

It is desirable to alloy iron, copper, and nickel to the baseline to accelerate corrosion. Nickel, copper, iron, or a combination of the three may be added to achieve a specific dissolution rate by intra-granular or intergranular galvanic corrosion. Copper alone will not have a sufficient corrosion rate for many conditions. Nickel and iron may drop out of solution if an improper amount is added. Tuning the corrosion rate without a detrimental impact to mechanical properties often requires a combination of two elements in a particular amount.

The present invention disclosure shows the dissolvable magnesium alloy compatible for the conditions associated with downhole operations, such as hydraulic fracturing operations. When the dissolvable magnesium alloy is formed in a component of a downhole tool, the component must have the same functionality as the conventional non-dissolving component. The component must be sufficiently strong to hold a pressure differential around 7.5 ksi as assembled in the downhole tool. There may be other components of the dissolvable magnesium alloy in the downhole tool as well. The component must also dissolve in a wellbore fluid, such as a potassium chloride brine, after the downhole operation is completed. The alloy remains strong, and ductile to be formed into a component and functional as a downhole tool. The dissolvability is controlled within a range for a potassium chloride brine. Additionally, the yield strength, ultimate tensile strength, and elongation of the present invention are sufficient to function as component of a downhole tool, despite the additives in grains of the magnesium to affect overall strength.

In one embodiment, gadolinium and yttrium in magnesium alloys are used to improve the mechanical properties (tensile strength and yield strength) of magnesium alloys. Elements of copper, nickel, gallium and indium are used to improve the solubility of various other metal elements. Moreover, copper, nickel, gallium, indium and silicon in magnesium alloys increase the reaction rate of magnesium alloys with a medium. Other elements in magnesium alloys, such as aluminum, zinc, zirconium, rhenium, iron, beryllium and calcium, may serve to catalyze the improvement of the mechanical properties of magnesium alloys.

As shown in FIG. 1 , AZ31 is used to compare to Alloys 1, 2, 3, 4, 5 and 6. In another word, AZ31 may be considered a baseline for magnesium alloy development. The alloy is 3 weight % aluminum, 1 weight % zinc, with the balance of magnesium. A yield of 30 ksi and UTS of 38 ksi are paired with an elongation of 15%.

The FIG. 1 demonstrates the low and high mechanical properties boundaries of aluminum content manipulation. First, by starting with a baseline alloy of Mg (alloy 1), 0.5% wt Al, 1.4% wt Cu, and 0.15% wt Ni, a yield of 18.2 ksi was achieved with an elongation of 8.1%. The elongation would be sufficient for a component of a downhole tool, but the yield strength may be lacking. To test the upper bounds of aluminum content, as shown in Table 1, Alloy 2 was created with 10% wt Al, 1.4% wt Cu, 1% wt Zn and 0.15% wt Ni, achieving a yield of 29.2 ksi and a ductility of 12.8%. The resultant alloy mirrors the strength of AZ31, but lacks the ductility. Then, setting the aluminum content at roughly half of Alloy 2, and unexpected result was measured in Alloy 3 in that the ductility exceeded both alloys by 50% with mechanical strength in line with the stronger alloy. A chemistry of 5.6% wt Al, 1.4% wt Cu, and 0.15% wt Ni achieved these surprising results.

TABLE 1 Composition of various alloys Alloy Composition Alloy 1 0.5% wt Al, 1.4% wt Cu, 0.15% wt Ni, and the balance of Mg (97.95 wt %) Alloy 2 10% wt Al, 1.4% wt Cu, 1% wt Zn, 0.15% wt Ni, and the balance of Mg (87.45%) Alloy 3 5.6% wt Al, 1.4% wt Cu, and 0.15% wt Ni, and the balance of Mg (92.85%) Alloy 4 0.5% wt Al, 1.4% wt Cu, 0.15% wt Ni, and the balance of Mg (97.95%) Alloy 5 3.1% wt Gd, 1.4% wt Cu, 0.15% wt Ni, and the balance of Mg (95.35%) Alloy 6 3.1% wt Gd, 1.4% wt Cu, 0.15% wt Ni, 4% wt Y, and the balance of Mg (91.35%) Alloy 7 3.3% wt Al, 1% wt Gd, 0.6% wt Y, 11% wt Li, 0.4% wt Ni, 0.2% wt Cu, and the balance of Mg (83.5%)

The yield and UTS of Alloy 3 fits in the window close to AZ31, but the ductility unexpected exceeded the base and modified chemistries. Then, again improving on Alloy 1, Alloy 4 was created by adding 3.1 wt % gadolinium with 0.5 wt % Al, 1.4 wt %) Cu, and 0.15 wt % Ni, achieving a remarkable increase in the yield strength to 37 ksi, a 100% increase, with ductility staying constant. Alloy 4 sees an unexpected increase in mechanical properties compared to alloy 1 after an addition of gadolinium.

Alloy 5 was created by removing the aluminum from alloy 4, with a formulation of 3.1 wt % Gd, 1.4 wt % Cu, and 0.15 wt % Ni having the startling effect of a dramatic increase in ductility to 16%, a 50% improvement over Alloy 4. The yield strength fell relative to alloy 4 to 23.8 ksi. The ductility for alloy 5 unexpectedly is increased while the yield and UTS is essentially unchanged from alloy 1 after the removal of aluminum and addition of gadolinium equal to alloy 4. Alloy 6 results in the increase of all properties with the addition of yttrium to alloy 5.

More specifically, alloy 6 improves upon Alloy 5 with the addition of 4 wt % Y, with 3.1 wt % Gd, 1.4 wt % Cu, and 0.15 wt % Ni. Again, the yield strength experienced a dramatic increase to 32.4 ksi, a 40% increase, while increasing the ductility to 17.5%. Yttrium unexpectedly optimized both mechanical strength and ductility in alloy 6.

FIG. 2 illustrates the measured room temperature longitudinal mechanical properties of alloy 1 through 6, demonstrating the variety of useful mechanical properties required in different downhole components. More specifically, FIG. 2 shows values of the yield strength, ultimate tensile strength, and elongation for the material at room temperature from specimens oriented in the longitudinal direction. These are further enumerated in Table 2 with the transverse mechanical properties at room temperature.

TABLE 2 Tabulated values of the yield strength, ultimate tensile strength, and elongation for the material at room temperature from specimens oriented in the longitudinal and transverse directions. Longitudinal Transverse Long. Long. Long. Trans. Trans. Trans. Yield UTS Elong. Yield UTS Elong. Alloy (ksi) (ksi) (%) (ksi) (ksi) (%) AZ31 29.0 37.0 15.0 Alloy 1 18.2 29.2 8.1 7.7 22.6 11.0 Alloy 2 29.2 42.1 12.8 17.4 28.2 3.9 Alloy 3 24.4 38.7 17.0 12.4 33.7 13.2 Alloy 4 37.0 40.0 8.0 33.1 38.5 8.1 Alloy 5 23.8 32.7 16.0 20.3 31.3 19.9 Alloy 6 32.4 46.6 17.5 22.6 37.6 14.0 Alloy 7 38.0

FIG. 3 shows the unexpected thermally stable mechanical properties of alloy 6. More specifically, FIG. 3 demonstrate the mechanical properties of alloy 6 as a function of temperature at 23, 95, 125, 150, and 175° C. Generally, the mechanical properties are stable from 23 to 175° C. Standard magnesium alloys experience a dramatic decrease in strength with an increase in temperature. It is a noteworthy result that there is less than a 10% decline in yield strength when measured from 23° C. to 175° C.

FIG. 4 contrasts the relative yield strength stability of alloy 6 to the alloys 1 through 5. Indeed, FIG. 4 shows a plot of the normalized yield strength at 23, 95, 125, 150, and 175° C. for alloys 1 through 6. Unlike alloy 6, most alloys including alloys 1-5 experience a significant decline in mechanical properties, even those with an equal amount of gadolinium. Yttrium unexpectedly acts as a thermal stabilizer for mechanical properties when gadolinium on its own was expected to. Because alloys 1 through 3 are lacking rare earth metals, they experience dramatic declines in yield strength. Alloys 4 and 5 demonstrate the typical moderate decrease in yield strength seen in magnesium-rare earth alloys, while alloy 6 only experiences a minimal decrease.

In another embodiment, alloy 7 is made with 3.3 wt % Al, 1 wt % Gd, 0.6 wt % Y, 11 wt % Li, 0.4 wt % Ni, and 0.2 wt % Cu. The lithium addition is to increase the ductility, as one skilled in the art would know. Unexpectedly, the ductility increased more than what a skilled practitioner would expect. FIG. 5 shows mechanical properties for the alloy 7 as a function of temperature, such as at 23, 95, 125, 150, and 175° C. The 36% ductility at room temperature increases to 60% at 95° C., a common frac plug utilization temperature. No other magnesium alloy can achieve this result.

FIG. 6 compares the room temperature ductility of alloy 7 to the others in this disclosure. Alloy 7 has shown a ductility of at least 100% more than any other alloy.

FIG. 7 compares the unique compressive properties of alloy 7 (right side) to two other high ductility materials, alloy 5 (left) and 6 (center), at 23° C. These display the standard 45° fracture face with little net geometry change. Alloy 7 showed high plasticity in that it is compressed to an unexpected 50% of original height with no cracking, a result that has not been seen in any other magnesium alloys.

FIG. 8 depicts the as-cast dissolution rate for the seven alloys compared to AZ31. A skilled practitioner recognizes that adding copper and nickel to magnesium may increase the dissolution rate, while rare earth and aluminum may decrease the corrosion rate. The nickel and copper content was essentially constant across the 7 materials. FIG. 8 shows the change in extruded dissolution rate by adding nickel, copper, aluminum, gadolinium in various combinations. By removing zinc and most aluminum, then adding nickel and copper from AZ31, alloy 1 was discovered to have a high dissolution rate. Increasing the aluminum to 10 wt % and adding 1 wt % zinc resulted in a lower dissolution rate for alloy 2. Alloy 3 was found to have a moderate corrosion rate due to the precipitation of the Q phase. The addition of 3.1 wt % Gd to alloy 1 resulted in an increase in the corrosion rate of the resulting alloy 4. Literature states that the addition of rare earth decreases the corrosion rate of the resultant magnesium alloy. It unexpectedly appears the combination of a small amount of aluminum, nickel, and/or copper with rare earth actually increases the corrosion rate. Aluminum is removed from alloy 4, dramatically decreasing the corrosion rate of alloy 5. Based on the results of alloy 4, it was unforeseen to have such a dramatic drop. A lower dissolution rate material is useful for high surface area components. Alloy 6 becomes more reactive when yttrium is added to alloy 5, an unexpected result. Lithium being added to alloy 4 results in a decrease in dissolution rate for Alloy 7. Surprisingly, alloy 3 has a lower dissolution rate than alloy 2, which has more aluminum. Further, alloy 4 has the unexpected result of an increased corrosion rate with the addition of rare earth. Even more astonishing was that the removal of aluminum from alloy 4 to create alloy 5 led to a dramatic drop in corrosion rate, where the skilled practitioner would anticipate a moderate increase. The addition of yttrium to alloy 5 to create alloy 6 led to another unexpected result of an increase in corrosion rate, where rare earth alloys are supposed to enhance corrosion resistance.

Still in FIG. 8 , an alloy 2 testing the upper bounds of aluminum content with the composition of 10 wt % aluminum, 1 wt % zinc, 1.5 wt % copper, and 0.1 wt % nickel is created. The alloy 2 has a yield of 29 ksi, UTS of 42 ksi, and an elongation of 13%. The mechanical properties are very similar to AZ31, but with a higher dissolution rate. A standard AZ31 alloy has Al₁₂Mg₁₇ form as a second phase, which forms in this case as seen in FIG. 9 c . Within the range of 0.5 to 10 wt % aluminum, there is no improvement over the baseline AZ31 material other than desired increased dissolution rate (57 mg/cm²/hr in 2.1 weight % KCl) due to the copper and nickel. The unique combination of copper and nickel formed 2% Q-phase (Al₇Cu₃Mg₆) which resulted in a moderate corrosion rate.

The strength of an alloy is contingent upon the ease with which dislocations move. Opposing dislocation motion increases mechanical strength. The addition of rare earth acts as a grain refiner in magnesium, as smaller grains hinder dislocation motion. Dislocations may be pinned due to stress field interactions with other dislocations and solute particles, creating physical barriers from second phase precipitates forming along grain boundaries. Further, rare earth depresses the corrosion rate. It is expected that corrosion will decrease significantly; the elongation would be similar, with a modest increase in yield and UTS.

The addition of 3.1 wt % gadolinium to 1.4 wt % copper, 0.6 wt % aluminum, and 0.15 wt % nickel (alloy 4) results in the formation of LPSO structures, specifically 14H. A yield strength of 37 ksi and UTS of 40 ksi is achieved with an elongation of 9%. Two important deviations from expectations occur. First, the yield doubled and secondly, the corrosion rate stays high (55 mg/cm²/hr in 2.1 weight % KCl). A modest increase in the UTS (40 ksi) of 33% is observed.

The combination of aluminum and gadolinium are known to act as a corrosion inhibitor. However, in this alloy, the corrosion rate increased from 45 to 55 mg/cm²/hr compared to embodiment without gadolinium.

Increasing the aluminum content and adding zinc (alloy 2) results in a dissolution rate of 57 mg/cm²/hr. Removing zinc and lowering aluminum to 5.6 weight % (Alloy 3) results in a dissolution rate of 45 mg/cm²/hr. Then, by adding 3.1 gadolinium, a dissolution rate of 55 mg/cm²/hr is measured in 2.1% KCl at 95° C. By removing aluminum, a dissolution rate of 13 mg/cm²/hr is measured in 2.1% KCl at 95° C. FIG. 3 demonstrates that copper and nickel clearly work as a corrosion accelerant, as will aluminum. In this particular set of alloys, gadolinium acts as both a corrosion inhibitor and accelerant.

In a final embodiment, aluminum is removed and 0.4 weight % zirconium is added with the expectation of a lower yield and UTS due to fewer phases generated, a modest increases in ductility, and a moderate decrease in dissolution rate. As expected, the yield decreased to 24 ksi, with UTS decreasing to 33 ksi. Surprisingly, the elongation is more than doubled to 19%, very similar to the 5.6 wt % Al alloy. No similar phases are formed between these two alloys, as seen in FIG. 9 e . However, a LPSO phase formed as verified from XRD, which instead results in a high elongation. Most unexpectedly, the dissolution rate dropped dramatically from 55 to 13 mg/cm²/hr in 2.1 weight % KCl. The phase are most similar to the Mg-1.4Cu-0.6Al-0.15Ni alloy with Mg₂Cu and Mg₂Ni, which had a very high dissolution rate.

FIG. 9 shows the scanning electron microscopy (SEM) images of the alloys, with an energy dispersive X-ray spectroscopy (EDS) overlay to show where elements concentrate for potential phase identification. As shown in FIG. 9 a , alloy 1 is at 1500× magnification via SEM on left and EDS at 3500× on right identifies several phases that accelerate corrosion. Mg₂Cu is observed in locations 1, 3, and 4. Mg₂Ni is found in locations 2 and 7. A complex phase of Ni, Cu, and Al is found in locations 5-6. No feature is observed in the microstructure that will contribute to higher mechanical properties than a base AZ31.

As shown in FIG. 9 b , alloy 2 is at 350× magnification via SEM on left. EDS at 3500× identifies Mg₁₇A112 in large quantities creating semi-enclosed cells that will reduce the rate of corrosion and increase the mechanical strength. Al₇Cu₃Mg₆ is formed which likely contributed to the higher corrosion rate. An AlMgZn tau phase is formed roughly double in proportion to the Al₇Cu₃Mg₆ phase. These secondary phases contribute to the increase in the mechanical properties. These formed rather than the Laves phases (Mg₂X) seen in alloy 1.

FIG. 9 c shows alloy 3 at 350× magnification via SEM on left. EDS at 3500× identifies Mg₁₇Al₁₂ in moderate quantities creating poorly-enclosed cells that will minimally delay corrosion while enhancing mechanical properties. The unique microstructure provides moderate mechanical strength with numerous second phases to pin dislocations, thus increasing the ductility. Location 1 shows where an AlNi phase precipitates, which will also increase corrosion rate. Location 2 is an instance of Al₇Cu₃Mg₆ phase, which contributes to the resultant mechanical properties.

Alloy 1 has fewer secondary phases than alloy 2. Alloy 3 had fewer secondary phases than Alloy 2, but a higher ductility. Alloy 5 has more secondary phases than alloy 3, with slightly lower ductility. The cuboidal features in the alloys formed from the copper and nickel addition act to increase the corrosion rate.

FIG. 9 d shows alloy 4 at 500× magnification via SEM on left, EDS at 5000× on right. FIG. 9 e shows alloy 5 at 350× magnification via SEM on left. EDS at 3500× identifies the locations of the LPSO phases at 1, 2, and 3. These are formed without the inclusion of zinc in the alloy, contrary to much of literature. Locations 4 and 5 show instances of the Mg₂Cu phase which contribute to accelerated corrosion.

FIG. 9 f shows alloy 6 at 350× magnification via SEM on left and EDS at 1500× on the right. FIG. 9 g illustrates Alloy 7 at 350× magnification via SEM on left. And EDS at 1500× on the right.

FIG. 10 shows an optical image of alloy 1, alloy 4, and alloy 5 at 100× and demonstrates the microstructure evolution from the alloying process. Taking alloy 1 as a baseline, adding Gd to create alloy 4 results in the formation of secondary phases which lend themselves to enhancing the mechanical properties. Removing the aluminum from alloy 1 while adding gadolinium results in alloy 5, with a new set of phases giving the high ductility measured in mechanical testing.

FIGS. 11 a-c show optical microstructures of alloy 1 (left), alloy 6 (center), and alloy with an extra yttrium (right) at 500× and demonstrates a boundary case of too much yttrium. Using Alloy 1 as a baseline, Alloy 6 is created with the addition of yttrium and gadolinium. Alloy 6 forms a network of semi-interconnected secondary phases with the addition of gadolinium and yttrium. Right (FIG. 11 c ) results from an increase in yttrium relative to gadolinium. Too much yttrium resulted in poor processing and properties. The semi-interconnected secondary phases lend themselves to high, thermally stable mechanical properties. Holding all other elements constant, the introduction of more yttrium created a large volume percentage of secondary phases, resulting in a strong but brittle alloy that had few uses in frac plugs.

FIGS. 12 a-d show the binary phase diagrams of magnesium and several alloying elements that a skilled practitioner would reference to anticipate solubility. More specifically, FIG. 12 a discloses binary phase diagram of magnesium and aluminum. As expected from this diagram, most but not all aluminum containing magnesium alloys formed the Mg₁₇Al₁₂ phase. A magnesium alloy with an aluminum content roughly between the maximum and minimum is created with the composition of 5.6 wt % aluminum, 0.4 wt % copper, and 1.4 wt % nickel (Alloy 3). Unexpectedly, both the longitudinal (21%) and transverse (13%) elongations increased to values higher than baseline AZ31 alloy. It is not inherent that aluminum will increase the elongation; one skilled in the art expects increasing aluminum content to decrease the ductility. For example, AZ91 has nominally 9 wt % aluminum with a ductility of 3-7%. The unique result is not expected by one skilled in the art. FIG. 9 c shows the open cells with small grains from which the high ductility emerges. From FIG. 12 a , there is no phase change from 5.6 to 10 wt % aluminum in a binary magnesium-aluminum phase diagram. Also, the yield is 25 ksi, with a high UTS (39 ksi). Typically, a magnesium alloy will have a high elongation or UTS, not both. A standard AZ31 alloy has Al₁₂Mg₁₇ form as a second phase, which also exists. The unique combination of copper and nickel formed 1% Q-phase (Al₇Cu₃Mg₆) which resulted in a moderate corrosion rate (45 mg/cm²/hr in 2.1 weight % KCl). Unexpectedly, no unique phase exists in this alloy compared to the former two cases that explain the high elongation.

FIG. 12 b teaches binary phase diagram of magnesium and zinc. The predicted zinc phase at low concentrations never materialize due to the alloying in alloy 2. Alloys other than alloys 3 through 5 may benefit from zinc addition based on the results from alloy 1 to 2. FIG. 12 c reveals binary phase diagram of magnesium and nickel. The predicted nickel phase at low concentrations precipitated in most cases, but also formed more complex phases that the skilled practitioner would not anticipate, leading to novel results. FIG. 12 d discloses binary phase diagram of magnesium and copper. The predicted copper phase at low concentrations precipitates in most cases, but also forms more complex phases that the skilled practitioner would not anticipate, leading to novel results.

FIG. 13 displays the X-Ray Diffraction (XRD) measurement of phases for the Mg-3Gd-1.4Cu in alloy 5 showing the creation of beneficial secondary phases. For example, the long-period stacking-order (LPSO 14H) structure will result in high strength and moderate ductility. To form LPSO phases, the rare earth elements must have negative mixing enthalpy not only with Mg, the hexagonal close-packed structure at room temperature, a large solid-solubility (>3.75 at. %) in Mg and an atomic size larger than Mg by 8%.

FIG. 14 displays the X-Ray Diffraction (XRD) measurement of phases for the Mg-3Gd-4Y-1.4Cu in alloy 6 showing the creation of beneficial secondary phases. For example, the long-period stacking-order structure will result in high strength and moderate ductility. To form LPSO phases, the rare earth elements must have negative mixing enthalpy not only with Mg, the hexagonal close-packed structure at room temperature, a large solid-solubility (>3.75 at. %) in Mg and an atomic size larger than Mg by 8%.

FIG. 15 shows the CALPHAD simulation for the Mg-3Gd-4Y-1.4Cu in alloy 6 with the mole fraction of phases formed during casting. Starting with a high temperature, the simulation predicts what elements/phases appear with dropping temperature. XRD work confirmed the presence of many of these alloys.

FIG. 16 shows alloy 4 and alloy 4 modification using the same weight percentage rare earth neodymium instead of gadolinium. It demonstrates that the claimed mechanical performances can be materially replicated by substituting in one rare earth for another. In this instance, alloy 4 is compared to a modified version with an identical chemistry except the gadolinium is removed and replaced with an equal amount of neodymium, for example.

All known dissolvable magnesium alloys start from a chemistry in the range of 0-20 wt % lithium, 0-15 wt % gadolinium, 0-15 wt % yttrium, 0-2 wt % copper, 0-2 wt %) nickel, 0-2 wt % zirconium, 0-15 wt % aluminum, and up to 10% total of other elements including but not limited to manganese, neodymium, cerium, calcium, iron, bismuth, indium, and silver with the balance magnesium. With all zero, plain elemental magnesium is the starting point.

From the testing Mg-3Gd-4Y-1.4Cu in alloy 6 is at least one embodiment of the dissolvable magnesium alloy of the present invention. The strength (tensile yield strength, ultimate tensile strength) and performance (elongation and dissolution rate) are compatible with components of a downhole tool. More specifically, one chemistry of Mg-3Gd-4Y-1.4Cu alloy has the form of 3.1 wt % gadolinium, 4 wt % yttrium, 1.4 wt % copper, wt % nickel, and 0.4 wt % zirconium with magnesium as the balance. This embodiment of the present invention achieves 32.4 ksi yield, 46.6 ksi UTS, 17.5% elongation, and a dissolution rate of 25 mg/cm²/hr in 95° C. 2.1 wt % KCl. Removing the copper and nickel from this chemistry may also form a corrosion resistant alloy for long duration applications.

Example 1

In one embodiment, magnesium alloy of the present invention comprises wt Al, 1.4% wt Cu, 0.15% wt Ni, 97.95 wt % Mg. In one embodiment, the production process is as follows:

Weighing raw materials such as magnesium, aluminum, copper, nickel, and pretreating magnesium, aluminum, copper, and nickel at 100 C for 5 h; mixing the raw materials, and then smelting them in a crucible electric resistance furnace, covering them with a covering agent, and refining them with a refining agent, thus the components are uniformly mixed, removing the inclusions, and casting the materials at 670 C to form an ingot; subjecting the ingot to a homogenization heat treatment at 450 C for a treatment time of 8 h; subjecting the ingot to a forging processing at 350 C so as to obtain a forged piece; subjecting the forged piece to an aging heat treatment at room temperature for a treatment time of 20 h.

The foregoing disclosure and description of the invention is illustrative and explanatory thereof. Various changes in the details of the illustrated structures, construction and method can be made without departing from the true spirit of the invention. 

1. A dissolvable alloy for a component of a downhole tool, comprising: from 0.04 to 0.4 wt % nickel; 1.4 wt %-2.0 wt % copper; 3.1 wt %-15 wt % gadolinium; 4.0 wt %-15 wt % yttrium; 0.5 wt %-3.3 wt % aluminum; and at least 83.5 wt % magnesium so as to be dissolvable in KCl at 2.1% by weight and 93° C. with a dissolving rate in a range of from 10 to 100 mg/cm²/hr, yield strength in a range of from 25 to about 37 ksi, ultimate tensile strength in a range of from 35 to 45 ksi, and elongation in a range of from 10 to 19%, wherein the dissolvable alloy does not have Zr.
 2. The dissolvable alloy of claim 1 further comprising 0-20% lithium by weight, 3.1% gadolinium by weight, 4% yttrium by weight, 1.4% copper by weight.
 3. The dissolvable alloy of claim 2, further comprising up to 10 wt % total of other elements. which comprises one of manganese, neodymium, cerium, calcium, iron, bismuth, indium, or silver.
 4. (canceled)
 5. The dissolvable alloy of claim 3, wherein said copper is 1.4 wt %, said aluminum is 10.1 wt %, said zinc is 0.45 wt %, said nickel is 0.15 wt %, and said manganese is 0.16 wt %.
 6. The dissolvable alloy of claim 2, wherein said copper is 0.4 wt %, said nickel is 0.04 wt %, and said aluminum is 0.5 wt %.
 7. The dissolvable alloy of claim 2, wherein said gadolinium is 3.1 wt %, said copper is 1.4 wt %, said nickel is 0.04 wt %, and said aluminum is about 0.5 wt %.
 8. The dissolvable alloy of claim 2, wherein said gadolinium is 3.1 wt %, said copper is 0.4 wt %, said nickel is 0.04 wt %, and said aluminum is 0.5 wt %.
 9. The dissolvable alloy of claim 2, wherein said copper is 0.4 wt %, said nickel is 0.15 wt %, and said aluminum is 5.6 wt %.
 10. The dissolvable alloy of claim 2, wherein said copper is 1.2 wt %, said nickel is 0.4 wt %, and zinc is 6.1 wt %.
 11. The dissolvable alloy of claim 2, wherein said gadolinium is 1.56 wt %, said nickel is 0.04 wt %, and wherein said zinc is 3.88 wt %.
 12. The dissolvable alloy of claim 2, wherein said lithium is 11 wt %, said gadolinium is 1.0 wt %, said yttrium is 0.6 wt %, said nickel is 0.4 wt %, said copper is 0.2 wt %, and wherein said zinc is 3.3 wt %.
 13. An alloy, comprising: 0-20% lithium by weight, 0-15% gadolinium by weight, 0-15% yttrium by weight, 0-2% copper by weight, 0-15% aluminum by weight, 0.04% to about 0.4% nickel by weight; the balance of magnesium (Mg) and inevitable impurities, so as to be dissolvable in KCl at 2.1% by weight and about 93° C. with a dissolving rate in a range of from 10 to 100 mg/cm²/hr, yield strength in a range of from 25 to 37 ksi, ultimate tensile strength in a range of from 35 to 45 ksi, and elongation in a range of from 10 to 19%.
 14. The alloy of claim 13, further comprising up to 10% total of other elements. which comprises one of manganese, neodymium, cerium, calcium, iron, bismuth, indium, or silver.
 15. The alloy of claim 13, wherein the copper to nickel ratio is within the range of 50:1 to 0.1:1 wt %.
 16. The alloy of claim 15, wherein said range of aluminum is 7 to 12 wt %, particularly when the range of aluminum to copper is in the range of 3.5:1 to 60:1 wt %.
 17. The alloy of claim 13, wherein said range of zinc is 0.5 to 3 wt %, particularly when the range of copper to zinc is in the range of 0.07:1 to 4:1 wt %.
 18. An alloy, comprising: 0.04% to about 0.4% nickel by weight; 0.2% to about 2% copper by weight; and the balance of magnesium (Mg) and inevitable impurities, wherein the alloy yields strength in a range of from 25 to 37 ksi, ultimate tensile strength in a range of from 35 to ksi, and elongation in a range of from about 10 to about 19%.
 19. The alloy of claim 18, further comprising lithium by weight, 0-15% gadolinium by weight, 0-15% yttrium by weight, 0-15% aluminum by weight.
 20. The alloy of claim 18, further comprising up to 10 wt % total of other elements. which comprises one of manganese, neodymium, cerium, calcium, iron, bismuth, indium, or silver. 